For the mobile communication, limited radio resources need to be shared by all users, so as to enable different users at different locations to communicate with each other simultaneously and reduce interference as possible. This is just the so-called multiple access technology.
Along with the rapid development of the radio communication, the numbers of users and the amount of services have grown explosively, which thereby puts forward higher requirement on a system capacity of a radio network continuously. Researches show that, the mobile traffic volume has been doubled every year, and in the year of 2020, there will be approximately 50, 000, 000, 000 user equipments (UEs) accessing to the radio mobile network. Due to such an explosive growth, the multiple access technology has become a core issue for the upgrade of the network. The multiple access technology may determine a basic capacity of the network, and may significantly affect the system complexity as well as the deployment cost.
An orthogonal multiple access technology, e.g., frequency-division multiple access technology, time-division multiple access technology, code-division multiple access technology or orthogonal frequency-division multiple access technology, may be adopted by the traditional mobile communication (1st Generation to 4th Generation).
Due to the transmission delay and signal multipath propagation, it is necessary to provide, during the design, a protection time for a time-division multiple access system, so as to ensure that the users are orthogonal to each other. Identically, due to the non-ideality of a filter, a bandwidth edge of a signal may not be reduced suddenly, and the out-of-band diffusion may inevitably occur for the signal. At this time, it is necessary to provide a protection bandwidth for a frequency-division multiple access system, so as to ensure the orthogonality. In addition, due to the signal multipath propagation, it is impossible for a code-division multiple access system to ensure the complete orthogonality among code words, which also leads to the capacity losses. For an orthogonal frequency-division multiple access system, there is certain bandwidth overlap among sub-channels so as to improve the band utilization. However, in order to ensure the orthogonality between symbols in a time domain, a Cyclic Prefix may be introduced, which also leads to the loss in system efficiency. The traditional mobile communication system is designed on the basis of the orthogonal multiple access technology and a linear receiver, so as to facilitate the transmission and reception for the system.
In a word, from the point of a multiuser information theory, it is merely able for an orthogonal mode to reach an inner bound of a multiuser capacity region, which results in relatively low radio resource utilization.